Conventional plastics are stable for a long period of time in natural environments and moreover have small bulk densities; problems have been pointed out that they renders shorter the service life of reclaimed lands for filling wastes there and damage natural view or life environments of wild animals and plants. Accordingly, materials that degrade and disappear with time in natural environments, thereby giving no adverse influences to the natural environments are called for. What has now attracted attention as such a material is a biodegradable resin. The biodegradable resins are known to be biodegraded in soil or water by hydrolysis or the action of microorganisms to finally produce harmless decomposed matter. It is also known that composted biodegradable resins can be readily disposed of.
Biodegradable resins that are going into practical application include aliphatic polyesters, modified PVA, cellulose ester compounds, modified starch, and blends of these. The aliphatic polyesters, which have a wide range of properties and processability close to that of general-purpose resins, have got into a wide use. For example, lactic acid based resins in combination with other aliphatic polyesters have excellent transparency, rigidity, heat resistance and so on and hence have attracted attention as a substitute material of, for example, polystyrenes and ABS resins in the field of injection molding such as appliance, OA devices and so on.
Materials used for appliance, OA devices and so on are required of flame retardance for preventing fire, and mainly halogene-based, particularly bromine-based flame retardants have been added to polystyrenes, ABS resins and soon. However, it is pointed out as the possibility that the halogen-based retardants generate noxious gases such as dioxine when they are burned. Accordingly, safety upon incineration of their wastes and thermal recycle is questioned. Phosphorus-based flame retardants have been developed to substitute for halogen-based flame retardants. There is the possibility that after the landfill operations, phosphorus is eluted from the wastes to contaminate soil and water; safety to humans and harmony with the environment is insufficient. Further, resins to which phosphorus-based flame retardants are added may be susceptible to adverse influences on practically important properties such as moldability, heat resistance and so on. To avoid the above-mentioned problems, flame retardants that contain neither halogen nor phosphorus have been under development. For example, metal hydroxides attract attention as having harmony with the environment, generating no noxious gases when they are disposed of.
However, when metal hydroxides are added to lactic acid based resins, degradation of the lactic acid based resins proceeds in the presence of water or the like, which causes a decrease in the molecular weight and mechanical strength of the lactic acid based resins and the like. To impart sufficient flame retardance to a resin, a large amount (for example, about 150 mass parts or more per 100 mass parts of resin) of a metal hydroxide must be added to the resin. With an increasing amount of the metal hydroxide used, the resin undergoes an increasing rate of a reduction in the mechanical strength thereof. Japanese Patent Application Laid-open No. Hei 8-252823 discloses addition of aluminum hydroxide or magnesium hydroxide to biodegradable plastics to impart flame retardance therewith. However, to obtain satisfactory flame retardance, it is necessary to add a large amount of, for example, magnesium hydroxide to the biodegradable plastics, resulting in a decrease in the mechanical strength of the plastics.
Japanese Patent Application Laid-open No. 2000-319532 discloses a flame retardant resin composition comprised by a resin and silicon oxide as a non-halogen-based and non-phosphorus-based flame retardant, which is made compatible with the resin. However, the resin composition does not have sufficient flame retarding performance, so that they find no use in appliance, OA devices and so on nor have sufficient mechanical properties.